During cell death or apoptosis, cells express phosphatidylserine. This compound has a serine group attached to two long hydrocarbon groups. It is known that annexin, or modified annexins, will preferentially bind to the serine of phosphatidylserine.
Accordingly, annexin has been used to target a variety of apoptotic cells for both therapeutic and imaging purposes. An appropriate therapeutic or imaging agent is attached to the annexin which then can be injected into a subject. In the bloodstream, the modified annexin will preferentially bind to the apoptotic cells, providing either a means to provide therapy to the individual or enable imaging of the apoptotic region.
By way of background, annexin is a protein with about 300 amino acids. There are a variety of different annexins. In particular, annexin-V, which has 319 amino acids, binds with high affinity to phosphatidylserine. The annexin binds to phosphatidylserine in a calcium dependent manner.
In order to utilize the annexin to deliver either the therapeutic or imaging agent to the apoptotic cell, the imaging agent or therapeutic compound, referred to as a payload, must be attached to the annexin. Annexin has multiple lysine groups that have pendent amine groups that are available for derivation. However, the ability of the annexin to attach to the phosphatidylserine is greatly diminished if too many lysine groups are derivatized. Thus, by labeling the annexin, its affinity to phosphatidylserine can be reduced.
In many applications, this may not be an extreme problem in that excess labeled annexin can be administered. However, in particular, with radiopharmaceuticals, the amount of radioactive material that can be administered to a patient is limited. Therefore, it is imperative that the binding affinity of the labeled annexin be maximized. This is also preferred in virtually any application that relies on annexin's affinity for phosphatidylserine.